Antibody (Ab) production and the presence of B cells within the central nervous system (CNS) is well documented in humans with the demyelinating disease multiple sclerosis (MS) and those afflicted by neurotropic infections. Their role in MS is currently unclear. However, detrimental humoral responses are implied by ongoing immune activation due to ectopic B cell follicle formation, as well as improvement in MS patients treated with anti-CD20 monoclonal Ab rituximab to reduce circulating B cells. Antibody secreting cells (ASC) are also detrimental if Ab are cross-reactive with, or directly target, self antigens. By contrast, during viral CNS infections intrathecal humoral responses are associated with protective functions. Locally produced anti-viral Ab coincide with sustained immune control within the CNS without overt pathology implicating a potent non lytic anti-viral role. Furthermore, the potential danger of losing control of clinically inapparent persisting viruses became apparent by development of progressive multifocal leukoencephalopathy following rituximab treatment during therapy for rheumatoid arthritis and MS. Despite the biological significance of humoral immunity in diverse settings of neuroinflammation, how Ab production in the CNS is sustained and which B cell populations and environmental factors support differentiation of ASC remain poorly characterized. This proposal uses a neurotropic coronavirus model of acute and persistent infection associated with demyelination to define the interactions between peripheral and CNS humoral responses in supporting CNS Ab production. The overall goal is to broaden insights into treatment options for inflammatory diseases such as MS, without provoking emergence of endogenous viruses, as well as targeting acute viral encephalitis. Three Specific Aims are pursued. Aim 1 will define the contribution of peripheral lymphoid tissue derived B cells in replenishing and maintaining ASC within the CNS. Results will reveal whether all B cells within the CNS are germinal center derived, and whether ASC migrating to the CNS differentiate within the CNS to become long lived ASC. Furthermore, the unexplored role of non Ab producing memory B cells (Bmem) to CNS humoral immunity will be defined. The role of antigen (Ag) and CD4 T cells in promoting B cell differentiation to sessile ASC within the CNS will be determined in Aims 2 and 3, respectively. Approaches are based on immunization of transgenic mice in which germinal center derived B cells are marked by fluorescence to isolate ASC and Bmem for adoptive transfer. Their CNS migration, differentiation and protective capacity will be monitored in recipients defective in Ab production. The influence of cognate Ag is examined by variations in donor B cell specificities, and inflammatory insult. T cell help will be assessed b CD4 T cell ablation and supplementation approaches. The results will for the first time define the parameters regulating both protective and pathogenic responses associated with B cells during human autoimmunity and CNS infections.